It occurs free in bitter almonds, being formed by an enzyme
decomposition of amygdalin (q.v.). It may also be prepared by oxidizing
benzyl alcohol with concentrated nitric acid; by distilling a mixture of
calcium benzoate and calcium formate; by the condensation of
chlor-oxalic ester with benzene in the presence of aluminium chloride,
the ester of the ketonic acid formed being then hydrolysed and the
resulting acid distilled:
C6H6 + Cl.CO.COOC2H5 = C6H5CO.COOC2H5 + HCl,
C6H5CO.COOH = C6H5CHO + CO2;
by the action of anhydrous hydrocyanic acid and hydrochloric acid on
benzene, an aldime being formed as an intermediate product:
C6H6 + HCN + HCl = C6H5CH : NH.HCl,
Benzaldine hydrochloride
C6H5CH : NH.HCl + H2O = NH4Cl + C6H5CHO;
and by the action of chromium oxychloride on toluene dissolved in carbon
bisulphide (A. Etard, _Berichte_, 1884, 17, pp. 1462, 1700).
Technically it is prepared from toluene, by converting it into benzyl
chloride, which is then heated with lead nitrate:
C6H5CH2Cl + Pb(NO3)2 = 2NO2 + PbCl.OH + C6H5CHO,
or, by conversion into benzal chloride, which is heated with milk of
lime under pressure.
C6H5CHCl2 + CaO = CaCL2 + C6H5CHO.
E. Jacobsen has also obtained benzaldehyde by heating benzal chloride
with glacial acetic acid:
C6H5CHCl2 + CH3COOH = CH3COCl + HCl + C6H5CHO.
Benzaldehyde is a colourless liquid smelling of bitter almonds. Its
specific gravity is 1.0636 (0/0 deg. C.), and it boils at 179.1 deg. C.
(751.3 mm). It is only slightly soluble in water, but is readily
volatile in steam. It possesses all the characteristic properties of an
aldehyde; being readily oxidized to benzoic acid; reducing solutions of
silver salts; forming addition products with hydrogen, hydrocyanic acid
and sodium bisulphite; and giving an oxime and a hydrazone. On the other
hand, it differs from the aliphatic aldehydes in many respects; it does
not form an addition product with ammonia but condenses to
hydrobenzamide (C6H5CH)3N2; on shaking with alcoholic potash it
undergoes simultaneous oxidation and reduction, giving benzoic acid and
benzyl alcohol (S. Cannizzaro); and on warming with alcoholic potassium
cyanide it condenses to benzoin (q.v.).
The oxidation of benzaldehyde to benzoic acid when exposed to air is not
one of ordinary oxidation, for it has been observed in the case of many
compounds that during such oxidation, as much oxygen is rendered
"active" as is used up by the substance undergoing oxidation; thus if
benzaldehyde is left for some time in contact with air, water and
indigosulphonic acid, just as much oxygen is used up in oxidizing the
indigo compound as in oxidizing the aldehyde. A. v. Baeyer and V.
Villiger (_Berichte_, 1900, 33, pp. 858, 2480) have shown that benzoyl
hydrogen peroxide C6H5.CO.O.OH is formed as an intermediate product and
that this oxidizes the indigo compound, being itself reduced to benzoic
acid; they have also shown that this peroxide is soluble in benzaldehyde
with production of benzoic acid, and it must be assumed that the
oxidation of benzaldehyde proceeds as shown in the equations:
C6H5CHO + O2 = C6H5.CO.O.OH,
C6H5CO.O.OH + C6H5CHO = 2C6H5COOH.
Further see G. Bodlander, _Ahrens Sammlung_, 1899, iii. 470; W.P.
Jorissen, _Zeit. fur phys. Chem._, 1897, 22, p. 56; C. Engler and W.
Wild, _Berichte_, 1897, 30, p. 1669.
The oxime of benzaldehyde (C6H5CH:N.OH), formed by the addition of
hydroxylamine to the aldehyde, exhibits a characteristic behaviour when
hydrochloric acid gas is passed into its ethereal solution, a second
modification being produced. The former (known as the [alpha] or
benz-anti-aldoxime) melts at 34-35 deg. C.; the latter ([beta] or
benz-syn-aldoxime) melts at 130 deg. C. and is slowly transformed into
the [alpha] form. The difference between the two forms has been
explained by A. Hantzsch and A. Werner (_Berichte_, 1890, 23, p. 11) by
the assumption of the different spatial arrangement of the atoms (see
STEREO-ISOMERISM). On account of the readiness with which it condenses
with various compounds, benzaldehyde is an important synthetic reagent.
With aniline it forms benzylidine aniline C6H5CH:N.C6H5, and with
acetone, benzal acetone C6H5CH : CH.CO.CH3. Heated with anhydrous sodium
acetate and acetic anhydride it gives cinnamic acid (q.v.); with ethyl
bromide and sodium it forms triphenyl-carbinol (C6H5)3C.OH; with
dimethylaniline and anhydrous zinc chloride it forms leuco-malachite
green C6H5CH[C6H4N(CH3)2]2; and with dimethylaniline and concentrated
hydrochloric acid it gives dimethylaminobenzhydrol,
C6H5CH(OH)C6H4N(CH3)2. Heated with sulphur it forms benzoic acid and
stilbene:
2C7H6O + S = C6H5COOH + C6H6CHS,
2C6H5CHS = 2S + C14H12.
Its addition compound with hydrocyanic acid gives mandelic acid
C6H5CH(OH).COOH on hydrolysis; when heated with sodium succinate and
acetic anhydride, phenyl-iso-crotonic acid C6H5CH : CH.CH2COOH is
produced, which on boiling is converted into [alpha]-naphthol C10H7OH.
It can also be used for the synthesis of pyridine derivatives, since A.
Hantzsch has shown that aldehydes condense with aceto-acetic ester and
ammonia to produce the homologues of pyridine, thus:
R R
| |
ROOC.CH2 CHO CH2.COOR ROOC.C-CH-C.COOR
| + + | = || || + 3H2O.
H3C.CO NH3 CO.CH3 H3C.C-NH-C.CH3
On nitration it yields chiefly meta-nitro-benzaldehyde, crystallizing in
needles which melt at 58 deg. C. The ortho-compound may be obtained by
oxidizing ortho-nitrocinnamic acid with alkaline potassium permanganate
in the presence of benzene; or from ortho-nitrobenzyl chloride by
condensing it with aniline, oxidizing the product so obtained to
ortho-nitrobenzylidine aniline, and then hydrolysing this compound with
an acid (_Farben fabrik d. Meister, Lucius und Bruning_). It
crystallizes in yellowish needles, which are volatile in steam and melt
at 46 deg. C. It is used in the artificial production of indigo (see
_German Patent_ 19768).
Para-nitrobenzaldehyde crystallizes in prisms melting at 107 deg. C. and
is prepared by the action of chromium oxychloride on para-nitrotoluene,
or by oxidizing para-nitrocinnamic acid. By the reduction of
ortho-nitrobenzaldehyde with ferrous sulphate and ammonia,
ortho-aminobenzaldehyde is obtained. This compound condenses in alkaline
solution with compounds containing the grouping -CH2-CO- to form
quinoline (q.v.) or its derivatives; thus, with acetaldehyde it forms
quinoline, and with acetone, [alpha]-methyl quinoline. With urea it
gives quinazolone
CH
/\ / \\N
| | |
| | CO ,
\/ \ /
NH
and with mandelic nitrile and its homologues it forms oxazole
derivatives (S.S. Minovici, _Berichte_, 1896, 29, p. 2097).
BENZENE, C6H6, a hydrocarbon discovered in 1825 by Faraday in the liquid
produced in the compression of the illuminating gas obtained by
distilling certain oils and fats. E. Mitscherlich prepared it in 1834 by
distilling benzoic acid with lime; and in 1845 Hofmann discovered it in
coal-tar. It was named "benzin" or "benzine" by Mitscherlich in 1833,
but in the following year Liebig proposed "benzol" (the termination _ol_
being suggested by the Lat. _oleum_, oil); the form "benzene" was due to
A.W. Hofmann. The word "benzine" is sometimes used in commerce for the
coal-tar product, but also for the light petroleum better known as
petroleum-benzine; a similar ambiguity is presented by the word
"benzoline," which is applied to the same substances as the word
"benzine." "Benzene" is the term used by English chemists, "benzol" is
used in Germany, and "benzole" in France.
Benzene is manufactured from the low-boiling fractions of the coal-tar
distillate (see COAL-TAR). The first successful fractionation of
coal-tar naphtha was devised by C.B. Mansfield (1819-1855), who
separated a benzol distilling below 100 deg. from a less volatile
naphtha by using a simple dephlegmator. At first, the oil was
manufactured principally for combustion in the Read-Holliday lamp and
for dissolving rubber, but the development of the coal-tar colour
industry occasioned a demand for benzols of definite purity. In the
earlier stages 30%, 50% and 90% benzols were required, the 30% being
mainly used for the manufacture of "aniline for red," and the 90% for
"aniline for blue." (The term "30% benzol" means that 30% by volume
distils below 100 deg.) A purer benzol was subsequently required for the
manufacture of aniline black and other dye-stuffs. The process
originally suggested by Mansfield is generally followed, the success of
the operation being principally conditioned by the efficiency of the
dephlegmator, in which various improvements have been made. The light
oil fraction of the coal-tar distillate, which comes over below 140 deg.
and consists principally of benzene, toluene and the xylenes, yields on
fractionation (1) various volatile impurities such as carbon disulphide,
(2) the benzene fraction boiling at about 80 deg. C., (3) the toluene
fraction boiling at 100 deg., (4) the xylene fraction boiling at 140
deg. The fractions are agitated with strong sulphuric acid, and then
washed with a caustic soda solution. The washed products are then
refractionated. The toluene fraction requires a more thorough washing
with sulphuric acid in order to eliminate the thiotolene, which is
sulphonated much less readily than thiophene.
Benzene is a colourless, limpid, highly refracting liquid, having a
pleasing and characteristic odour. It may be solidified to rhombic
crystals which melt at 5.4 deg. C. (Mansfield obtained perfectly pure
benzene by freezing a carefully fractionated sample.) It boils at 80.4
deg., and the vapour is highly inflammable, the flame being extremely
smoky. Its specific gravity is 0.899 at 0 deg. C. It is very slightly
soluble in water, more soluble in alcohol, and completely miscible with
ether, acetic acid and carbon disulphide. It is an excellent solvent for
gums, resins, fats, &c.; sulphur, phosphorus and iodine also dissolve in
it. It sometimes separates with crystals of a solute as "benzene of
crystallization," as for example with triphenylmethane, thio-p-tolyl
urea, tropine, &c.
Benzene is of exceptional importance commercially on account of the many
compounds derivable from it, which are exceedingly valuable in the arts.
Chemically it is one of the most interesting substances known, since it
is the parent of the enormous number of compounds styled the "aromatic"
or "benzenoid" compounds. The constitution of the benzene ring, the
isomerism of its derivatives, and their syntheses from aliphatic or
open-chain compounds, are treated in the article CHEMISTRY. A summary of
its chemical transformations may be given here, and reference should be
made to the articles on the separate compounds for further details.
Passed through a red-hot tube, benzene vapour yields hydrogen, diphenyl,
diphenylbenzenes and acetylene; the formation of the last compound is an
instance of a reversible reaction, since Berthelot found that acetylene
passed through a red-hot tube gave some benzene. Benzene is very stable
to oxidants, in fact resistance to oxidation is a strong characteristic
of the benzene ring. Manganese dioxide and sulphuric acid oxidize it to
benzoic and o-phthalic acid; potassium chlorate and sulphuric acid
breaks the ring; and ozone oxidizes it to the highly explosive white
solid named ozo-benzene, C6H6O6. Hydriodic acid reduces it to
hexamethylene (cyclo-hexane or hexa-hydro-benzene); chlorine and bromine
form substitution and addition products, but the action is slow unless
some carrier such as iodine, molybdenum chloride or ferric chloride for
chlorine, and aluminium bromide for bromine, be present. It is readily
nitrated to nitrobenzene, two, and even three nitro groups being
introduced if some dehydrator such as concentrated sulphuric acid be
present. Sulphuric acid gives a benzene sulphonic acid.
BENZIDINE (DIPARA-DIAMINO-DIPHENYL), NH2.C6H4.C6H4.NH2, a chemical base
which may be prepared by the reduction of the corresponding
dinitro-diphenyl, or by the reduction of azo-benzene with tin and
hydrochloric acid. In this latter case hydrazo-benzene C6H5NH.NH.C6H5 is
first formed and then undergoes a peculiar re-arrangement into benzidine
(see H. Schmidt and G. Schultz, _Annalen_, 1881, 207, p. 320; O.N. Witt
and Hans v. Helmont, _Berichte_, 1894, 27, p. 2352; P. Jacobson,
_Berichte_, 1892, 25, p. 994). Benzidine crystallizes in plates (from
water) which melt at 122 deg. C., and boil above 360 deg. C., and is
characterized by the great insolubility of its sulphate. It is a di-acid
base and forms salts with the mineral acids. It is readily brominated
and nitrated; when the nitration is carried out in the presence of
sulphuric acid, the nitro-groups take up the meta position with regard
to the amino-groups. Benzidine finds commercial application since its
tetrazo compound couples readily with amino-sulphonic acids, phenol
carboxylic acids, and phenol and naphthol-sulphonic acids to produce
substantive cotton dyes (see DYEING). Among such dyestuffs are
chrysamine or flavophenine, obtained from salicylic acid and diazotized
benzidine, and congo red obtained from sodium naphthionate and
diazotized benzidine. On the constitution of benzidine see G. Schultz
(_Annalen_, 1874, 174, p. 227).
_The Benzidine and Semidine Change._--Aromatic hydrazo compounds which
contain free para positions are readily converted by the action of
acids, acid chlorides and anhydrides into diphenyl derivatives; thus, as
mentioned above, hydrazo-benzene is converted into benzidine, a small
quantity of diphenylin being formed at the same time. The two products
are separated by the different solubilities of their sulphates. This
reaction is known as the _benzidine transformation_. If, however, one of
the para positions in the hydrazo compound is substituted, then either
diphenyl derivatives or azo compounds are formed, or what is known as
the _semidine change_ takes place (P. Jacobson, _Berichte_, 1892, 25, p.
992; 1893, 26, p. 681; 1896, 29, p. 2680; _Annalen_, 1895, 287, p. 97;
1898, 303, p. 290). A para mono substituted hydrazo compound in the
presence of a hydrochloric acid solution of stannous chloride gives
either a para diphenyl derivative (the substituent group being
eliminated), an ortho-semidine, a para-semidine, or a diphenyl base,
whilst a decomposition with the formation of amines may also take place.
The nature of the substituent exerts a specific influence on the
reaction; thus with chlorine or bromine, ortho-semidines and the
diphenyl bases are the chief products; the dimethylamino, -N(CH3)2, and
acetamino, -NHCOCH3, groups give the diphenyl base and the para-semidine
respectively. With a methyl group, the chief product is an
ortho-semidine, whilst with a carboxyl group, the diphenyl derivative is
the chief product. The ortho- and para- semidines can be readily
distinguished by their behaviour with different reagents; thus with
nitrous acid the ortho-semidines give azimido compounds, whilst the
para-semidines give complex diazo derivatives; with formic or acetic
acids the ortho-semidines give anhydro compounds of a basic character,
the para-semidines give acyl products possessing no basic character. The
carbon disulphide and salicylic aldehyde products have also been used as
means of distinction, as has also the formation of the stilbazonium
bases obtained by condensing ortho-semidines with benzil (O.N. Witt,
_Berichte_, 1892, 25, p. 1017).